In the albino rat, topical betamethasone 17-valerate acts as an anticorticosteroid. This steroid is inactive in a dermal atrophy assay over a dose range where betamethasone and hydrocortisone display atrophogenic activity. At appropriate concentrations betamethasone 17-valerate competitively inhibits the atrophogenic effects of both betamethasone and triamcinolone acetonide. Since betamethasone and betamethasone 17-valerate penetrate rat skin in vivo at essentially the same rate, it is concluded that the latter compound is relatively resistant to hydrolysis during penetration, and that it binds to rat corticosteroid receptor proteins in such a manner as to prevent expression of corticosteroid activity. Therefore, the rat cannot be used as a model species to predict activity in man for this compound.